One of the more common treatments for diseased or damaged hearts which are incapable of producing electrical excitation pulses, i.e., the pulses necessary to begin contraction of the heart's ventricles and atria, is to implant an internal cardiac pacemaker. Internal cardiac pacemakers electrically supply the necessary excitation pacing pulses directly to the heart through a set of electrodes. Such pacing pulses may be applied continuously if the heart is completely malfunctioning, or intermittently if it is only occasionally malfunctioning. Monitoring an electrocardiogram (ECG) signal from a patient having an internal pacemaker is difficult because pacing pulses generated by the internal pacemaker can interfere with the ECG signal and result in misdiagnosis of the heart's condition. For example, when an internal pacing pulse is generated, the QRS complex of the EGG signal is widened, which may falsely indicate acute myocardial infarction (AMI), a heart condition for which active therapy such as thrombolytic therapy is delivered. However, if the pacing pulse were to go undetected, a shockable heart rhythm could be mistakenly diagnosed by a ventricular fibrillation detection algorithm and a defibrillation shock applied. As yet another example, the ECG signal of a patient with an internal pacemaker who is experiencing asystole may contain periodically occurring pacing pulses. However, asystole is the cessation of heart activity and is indicated in the ECG signal by the absence of a QRS complex. Consequently, the presence of the pacing pulse may cause the asystole to go undetected.
Internal pacemakers are now much smaller in size and use far less energy to generate the pacing pulses. Accordingly, the pacing pulses are significantly smaller in both amplitude and duration, making them difficult to detect. It is even more difficult to detect such small pacing pulses in the presence of noise, which may essentially drown out the small pacing pulse entirely. As a result, many known internal pacing pulse detection techniques are no longer adequate for detecting pacing pulses. Hence, the threat of misdiagnosis remains.
Accordingly, a method and apparatus for detecting internal pacemaker pulses which are small in both amplitude and duration are needed. The method and apparatus should detect internal pacing pulses in the presence of noise with acceptable degrees of specificity and sensitivity. Further, the method and apparatus should provide a flexible range of energy values in which to detect a pacing pulse, so that as pacemaker technology advances, and pacing pulses become smaller, the method and apparatus of the present invention remains viable. As explained in the following, the present invention provides a method and apparatus that meets these criteria and solves other problems in the prior art.